Accelerometers are electromechanical devices that are widely used to measure acceleration forces due to motion and/or vibration. Capacitive accelerometers may find use in applications including seismic sensing, vibration sensing, inertial sensing and tilt sensing. Capacitive accelerometers are typically implemented as micro electromechanical systems (MEMS) and may be manufactured from a semiconductor material such as silicon. A typical MEMS sensing structure for a capacitive accelerometer comprises a proof mass moveably mounted to a support, with a set of electrode fingers extending from the proof mass being interdigitated with one or more sets of fixed electrode fingers so as to form a differential capacitor. WO 2004/076340 and WO 2005/083451 provide examples of capacitive accelerometers comprising a plurality of interdigitated fixed and moveable electrode fingers extending substantially perpendicular to the sensing direction of the MEMS device. The electrodes of the sensing structure are connected to suitable drive and pickoff electronics.
In a closed loop configuration, the electronics are arranged to drive the fixed electrode fingers with modulated voltage signals which produce variable electrostatic forces to balance the inertial forces so that the proof mass does not move under acceleration. This is achieved using a feedback signal to adjust the drive signals. WO 2004/076340 provides an example of an accelerometer operated in a closed loop configuration using pulse width modulation (PWM) of the drive signals. The mark:space ratio of the PWM signals can be adjusted to produce a variable rebalance force. Although sensitivity can be controlled in closed loop accelerometers, the necessary drive electronics and feedback loop increase the complexity and cost of the device.
In open loop operation of an accelerometer, the proof mass is free to move under acceleration, and this movement produces an output (“pickoff”) signal which can be rectified to produce a voltage representing the acceleration. The electronics are arranged to drive the fixed electrode fingers with a constant sine or square wave signal. Open loop accelerometers are often chosen for their simplicity in design to provide a low cost device.
The sensitivity of an accelerometer is the ratio of change in acceleration to change in the output signal. The sensitivity of the accelerometer to acceleration is assumed to be constant, such that a set signal (and therefore a set movement) represents a set acceleration. However, the actual sensitivity may change during use. For example, if the proof mass were to become mechanically damaged during use, the sensitivity of the accelerometer would change. An accelerometer is a spring mass system, where deflection is dependent on both the mass (of the proof mass and moveable fingers) and the stiffness of the support legs. The output voltage depends on the displacement of the proof mass and the gain of the accelerometer (e.g. the number and size of the moveable fingers), meaning that there are two ways in which an error can be introduced into the output.
For example, if an electrode finger or support leg for the proof mass were to crack, the electrical output or the amount of deflection produced by a set acceleration would be altered, both by the displacement changing due to the changed stiffness of the finger/leg, and due to the gain changing. However, the electronics would not be aware of the change in electrical or mechanical sensitivity, and would therefore produce incorrect measurements. It is therefore desirable to be able to test the sensitivity of an accelerometer during use.
In current open loop accelerometers, it is possible to perform a check on the electronics in the acceleration sensing system, e.g. in order to check the integrity of the electrical pickoff signal. However, this check is carried out downstream of the physical measurement, so cannot take into account the mechanical sensitivity of the accelerometer having changed.
The required sensitivity of an accelerometer depends on the application of the accelerometer, and the g-range in use. In low-g applications, changes in the sensitivity would have a greater impact. For example, in automotive applications, accelerometers may be used in determining when airbags are needed. If an accelerometer has a cracked support leg/spring (or other mounting for the proof mass), it may cause a large deflection for only a small acceleration, causing the airbags to be activated unnecessarily. This can be dangerous, and therefore it is important that the sensitivity of the accelerometer is known, and any changes in sensitivity can be detected during use.
The present disclosure seeks to provide sensitivity testing for capacitive accelerometers operating in an open loop configuration.